Devices which rely on the smooth continuous operation of a piston or shaft moving relative to a sleeve which surrounds at least a portion of the outer diameter of the piston or shaft have found many applications. For example, fluid control devices such as metering pumps for delivering precisely measured aliquots of fluid to a fluid outlet have found many uses in research, diagnostic testing and industry. Over the years, the wide variety of fluids delivered by these metering pumps has dictated the use of different compositions in the fabrication of the operative pump elements. The piston and the sleeve, which form the pumping chamber, must be able to function properly with the particular fluid being pumped. Pump designers have crafted pumps using numerous materials in order to keep pace with the growing needs of science. As these needs continue to grow, pump designers typically continue to search for new materials which will meet the needs of new pumping applications.
In addition to the use of new materials for different fluid applications, the technology driven desire to deliver smaller and more precisely controlled aliquots of fluids has provided an incentive for pump designers to reduce the clearances between the pumping elements, i.e. the piston and the sleeve. Those skilled in the art appreciate that in order to reduce the undesired leakage of fluid between the piston and sleeve, the clearance between these pumping elements is preferably minimized. For example, piston to sleeve clearances of about 0.00005-0.0002 are preferred for certain pump applications which require very precise volumetric control over a range of differential pressures in order to aoid syphoning through the pump. Those skilled in the art will appreciate that relatively small pressure differentials between the inlet and outlet of a pump can cause leakage of fluid through a pump if the pump is not constructed with tight enough clearances. Even small leakages through the pump are unacceptable for certain applications, e.g. pharmaceutical applications, requiring high degrees of precision.
While a variety of corrosion resistant materials have been successfully utilized for different pump applications, the use of smaller gap clearances has posed new problems to pump designers. Specifically, it has now been found that certain materials which had been successfully utilized with greater clearances will cause binding between the piston and sleeve when used with certain fluids and smaller clearances. Binding results when fluids cause substances within the pumping elements to leach from the pumping elements, react with the fluid, possibly precipitate, and then coagulate between the moving elements. It is believed that while the greater relative movement between moving elements which accompanies greater clearances is sufficient to prevent binding, smaller clearances allow significantly less relative movement which is insufficient to clear the coagulum. While explained herein in relation to a metering pump, this problem may occur in other devices which rely upon a piston or shaft moving within a sleeve with a tight clearance.
In light of this problem, there is a need to provide a cylinder and sleeve combination which is capable of functioning satisfactorily even when coagulum is building up between the moving elements.